Figure 19.3.3: F⁡(ϕ,k)
as a function of k2 and sin2⁡ϕ
for -1≤k2≤2, 0≤sin2⁡ϕ≤1.
If sin2⁡ϕ=1 (≥k2), then
the function reduces to K⁡(k),
becoming infinite when k2=1.
If sin2⁡ϕ=1/k2 (<1), then it has the value
K⁡(1/k)/k: put c=k2 in (19.25.5) and
use (19.25.1).